Marcio Mafra

Argon-Oxygen Post-Discharge Treatment of Hexatriacontane: Heat Transfer between Gas Phase and Sample

Characterization of the interaction between an argon-oxygen post-discharge and hexatriacontane (C36H74) is carried out. Optical emission spectroscopy using the atmospheric band O2 (b1Σg +, v=0 X3Σg −, v’=0) at 760 nm gives simultaneously the evolution of the O(3P) concentration above the surface and the gas temperature by simulation of the rotational spectrum of the transition. Surface reactions contribute to the heating in the sample and to a substantial increase in the gas temperature. Finally, a strong correlation between the time evolutions of the transition intensity and the sample temperature is observed, suggesting that O(3P) is the main reactive species that produces the heating and the chemical changes in the HTC.

Low-temperature plasma nitriding of sintered PIM 316L austenitic stainless steel

This work reports experimental results on sintered PIM 316L stainless steel low-temperature plasma nitriding. The effect of treatment temperature and time on process kinetics, microstructure and surface characteristics of the nitrided samples were investigated. Nitriding was carried out at temperatures of 350, 380, 410 and 440 oC , and times of 4, 8 and 16 h, using a gas mixture composed by 60% N2 + 20% H2 + 20% Ar, at a gas flow rate of 5.00 × 10-6 Nm3s-1, and a pressure of 800 Pa. The treated samples were characterized by scanning electron microscopy, X-ray diffractometry and microhardness measurements. Results indicate that low-temperature plasma nitriding is a diffusion controlled process. The calculated activation energy for nitrided layer growth was 111.4 kJmol-1. Apparently precipitation-free layers were produced in this study. It was also observed that the higher the treatment temperature and time the higher is the obtained surface hardness. Hardness up to 1343 HV0.025 was verified for samples nitrided at 440 oC. Finally, the characterization of the treated surface indicates the formation of cracks, which were observed in regions adjacent to the original pores after the treatment.

Influence of neutral and charged species on the plasma degradation of the stearic acid

In this work, stearic acid (SA) was degraded in an Ar-O2and Ar-H2post-discharge environment created by a plasma reactor with a microwave source and in an Ar, Ar-H2and Ar-O2DC (Direct Current) discharge environment created in a cathode-anode confined system. The afterglow region is useful for understanding the role of the chemically active species (O, O2, H and H2). In contrast, the discharge region allows the observation of the effects of chemically active species, charged species (ions and electrons) and photons. The influence of these species on the grafting and etching of SA was evaluated by measuring the mass variation, mass variation rate and chemical composition. The results showed that when only chemically active oxygen species are present, the SA is preferentially grafted. However, when both photons and charged species are present, the SA is more efficiently etched. When the Ar-H2and Ar environments are utilized; the SA is not efficiently degraded.

Plasma Sintering: A Novel Process for Sintering Metallic Components

Metallic components have been sintered in the presence of an abnormal glow discharge. The sample, which works as the cathode of the discharge, is heated by the bombardment of strongly accelerated ions and fast neutrals created in the cathode sheath. The components are sintered at 1120 °C during 20 minutes. The discharge is generated in an atmosphere containing either a mixture of 80% Ar + 20% H 2 or pure Ar at 2000 Pa. The temperature is adjusted by varying the average voltage of the power supply. In this work, microstructural results of unalloyed iron samples sintered by plasma are presented. Measurements of bulk porosity show that the plasma sintering process is activated as compared to the conventional furnace sintering. A lower porosity is also observed near the surface than in the bulk of the plasma-sintered sample. Such an effect is attributed to the sputtering that occurs due to the bombardment of the sample.

Evaluation of sample temperature and applied power on degradation of stearic acid in inductively coupled radio frequency plasma

Plasma cleaning is a promising technology in surface treatments, despite technological interest its use is limited because its mechanisms still are not entirely understood. This work aims to evaluate how the applied power of an inductively coupled RF discharge at 13,56 MHz, with Ar and Ar+10%O2 atmospheres, affects its capabilities to etch an organic molecule. Mass variation rate was used as direct characterization of degradation process and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) was performed to search for residual molecular modifications. Additionally, optical emission spectroscopy (OES) measurements were performed to monitor the offer of active species in the gaseous volume. In experimental conditions was possible attain mass reduction from sample, with higher mass loss rate when applied power is increased. Material characterization shows the possibility of attain a high etch rate, while no structural modifications were detected, if the temperature is controlled.

Plasma Etching of Stearic Acid in Ar and Ar-O2 DC Discharges

Stearic acid is treated in a DC Ar-O2 plasma created by a cathode-anode confined system. The influence of the most important process parameters (gas flow rate, sample temperature, output power and exposure time) on the acid modification is studied. The evaluation of the influence of these parameters on grafting and etching of stearic acid was done by measuring the mass variation rate (MVR). The results show that when charged and chemically active species increase in density, what is directly connected with plasma parameters, the MVR increases too. In all experimental conditions, a negative MVR was obtained, due to the etching of the sample. The etching rate decreases with processing time, probably because of the formation of a product which is more resistant to plasma etching.

Low-temperature Thermochemical Treatments of Stainless Steels – An Introduction

Plasma technology used to perform thermochemical treatments is well established for the majority of steels, but it is not the case for the different stainless steel classes. Thus, im‐ portant scientific and technological achievements can be expected in the coming years re‐ garding plasma-assisted thermochemical treatment of such steels. The metallurgical aspects as well as the application cost-efficiency of stainless steels impose specific re‐ quirements for the thermochemical treatment, such as easy native chromium-rich oxide layer removal and surface activation at low temperature, which do not appear for other steel classes (plain, low-alloy, and tool steels). Thus, due to the highly reactive physico‐ chemical environment created by the plasma, plasma-assisted technology presents ad‐ vantages over other “conventional” technologies like those performed in gas or liquid environments. Low temperature is needed to avoid the reduction of corrosion resistance of stainless steels, by suppressing chromium carbide/nitride precipitation, and, in this case, good surface properties are achieved by the formation of treated layers containing metastable phases. Such attributes make the low-temperature plasma thermochemical treatments of stainless steels an important R&D field in the domain of plasma technology and surface treatments, and the goal of this chapter is to introduce the reader to this im‐ portant topic.